Flow: Un-reachable code in disjoint union case analysis should trigger an error

[madnl]

Consider the following definitions for a disjoint union.

type Type1 = { tag: 'T1', value: string };
type Type2 = { tag: 'T2' };
type Type = Type1 | Type2;

and the function

const fn = (x: Type): string => {
    if (x.tag === 'T0') {
        return x.value;
    } else if (x.tag === 'T2') {
        return 'default';
    } else {
        return 'unexpected';
    }
}

Notice that in the first branch we're mistakingly using T0 instead of T1. This code will compile just fine.

I would expect flow to either

• raise an error telling me x.value in the first return leads to an invalid property access
• raise an error telling me the first return is an un-reachable code path because x.tag === 'T3' can never be true.

Either errors should lead to the realization that I've typo-ed the tag name in the first if.

Similarly, the following function which should compile fine raises a compilation error:

const fn = (x: Type): string => {
    if (x.tag === 'T1') {
        return x.value;
    } else if (x.tag === 'T2') {
        return 'default';
    }
}

Error:

const fn = (x: Type): string => {
                      ^^^^^^ string. This type is incompatible with an implicitly-returned undefined.

To fix this requires adding an additional else where we need to return some bogus value just to make the compiler happy:

const fn = (x: Type): string => {
    if (x.tag === 'T1') {
        return x.value;
    } else if (x.tag === 'T2') {
        return 'default';
    } else {
        return 'unexpected';
    }
}

This error also occurs when using switch statements instead of ifs.

[asolove]

I don't have a great answer, but there is a known hacky workaround to get exhaustiveness checking, described here: http://www.adamsolove.com/js/flow/type/2016/04/15/flow-exhaustiveness.html

[avikchaudhuri]

This will be fixed soonish.

[guicar]

We use another workaround to check the exhaustiveness of the switch: http://ouicar.github.io/2016/08/08/exhaustive-switch.html The idea is to add an empty type to make sure that the destructured value is of type any.

[calebmer]

This should be fixed in the latest version of Flow. If you are still having a problem with this please reopen with a reproduction on https://flow.org/try 😊

[asolove]

The example listed above seems to still have the problem on v51.1, which is the latest on try flow: https://flow.org/try/#0C4TwDgpgBAKuEEYoF4oG8rAIYHMBcUA5DAoQDRQBuWANgK4QEDOwATgJYB2OUAvgNwAoUJFjwATCnSZcBYuMJ8hI6HFGo1iKAB8xkcUMEBjAPacWUAGacpACgAeBTQEpmbLj2QA+dIKj+odksoBwA6bE9kVGIABkJnXwCkqFYIYDpWG3tQ6noIISTeKAgaJmggkOyIlCiiGAUEtD9k-1T0zKIAEwhLLDoaYEICgKKSssSWlLSMm0I6Tgh7SCNgCE6h5v9eQW2gA

Note that in this case, "has the problem" means that that code compiles fine, when OP thinks it should fail.